Stephen J. Romaniello

Rapidly assessing changes in bone mineral balance using natural stable calcium isotopes

The ability to rapidly detect changes in bone mineral balance (BMB) would be of great value in the early diagnosis and evaluation of therapies for metabolic bone diseases such as osteoporosis and some cancers. However, measurements of BMB are hampered by difficulties with using biochemical markers to quantify the relative rates of bone resorption and formation and the need to wait months to years for altered BMB to produce changes in bone mineral density large enough to resolve by X-ray densitometry. We show here that, in humans, the natural abundances of Ca isotopes in urine change rapidly in response to changes in BMB. In a bed rest experiment, use of high-precision isotope ratio MS allowed the onset of bone loss to be detected in Ca isotope data after about 1 wk, long before bone mineral density has changed enough to be detectable with densitometry. The physiological basis of the relationship between Ca isotopes and BMB is sufficiently understood to allow quantitative translation of changes in Ca isotope abundances to changes in bone mineral density using a simple model. The rate of change of bone mineral density inferred from Ca isotopes is consistent with the rate observed by densitometry in long-term bed rest studies. Ca isotopic analysis provides a powerful way to monitor bone loss, potentially making it possible to diagnose metabolic bone disease and track the impact of treatments more effectively than is currently possible.

Uranium isotopes fingerprint biotic reduction

Significance Throughout Earth’s history, redox transformations in sedimentary environments have occurred through chemical processes (abiotic pathways) or via the activity of living microorganisms (biotic pathway). Tools able to discriminate between these two mechanisms are of major interest, as they would contribute significantly to the understanding of biogeochemical events that shaped the evolution of life on our planet. Here, we show that there is a clear difference between the isotopic signature associated with abiotic and biotic transformations of uranium (U). Thus, U isotopic composition can serve as a marker for biological processes in many sedimentary rocks. Based on this result, we conclude that microbial activity has contributed to reductive sedimentary processes in many low-temperature redox-active terrestrial and marine environments. Knowledge of paleo-redox conditions in the Earth’s history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth’s crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium.

Global-ocean redox variation during the middle-late Permian through Early Triassic based on uranium isotope and Th/U trends of marine carbonates

Uranium isotopes (238U/235U) in carbonates, a proxy for global-ocean redox conditions owing to their redox sensitivity and long residence time in seawater, exhibit substantial variability in the Daxiakou section of south China from the upper-middle Permian through the mid-lower Triassic (∼9 m.y.). Middle and late Permian ocean redox conditions were similar to that of the modern ocean and were characterized by improving oxygenation in the ∼2 m.y. prior to the latest Permian mass extinction (LPME), countering earlier interpretations of sustained or gradually expanding anoxia during this interval. The LPME coincided with an abrupt negative shift of >0.5‰ in δ238U that signifies a rapid expansion of oceanic anoxia. Intensely anoxic conditions persisted for at least ∼700 k.y. (Griesbachian), lessening somewhat during the Dienerian. Th/U concentration ratios vary inversely with δ238U during the Early Triassic, with higher ratios reflecting reduced U concentrations in global seawater as a consequence of large-scale removal to anoxic facies. Modeling suggests that 70%–100% of marine U was removed to anoxic sinks during the Early Triassic, resulting in seawater U concentrations of <5% that of the modern ocean. Rapid intensification of anoxia concurrent with the LPME implies that ocean redox changes played an important role in the largest mass extinction event in Earth history.

Fully automated chromatographic purification of Sr and Ca for isotopic analysis

We present a commercially-available, fully-automated, offline chromatography method capable of simultaneously purifying both Ca and Sr for stable and radiogenic isotope analysis. The method features effective purification and mutual separation of Ca and Sr from complex matrixes using a single, highly-reusable chromatographic column. Low carryover combined with high yield for multiple extractions indicates the column can be reused for at least 200 samples. Accurate and precise stable and radiogenic isotope data are presented for BCR-2 basalt, NIST-1400 bone ash, IAPSO seawater, and an in-house llama bone standard (CUE-0001). The Sr–Ca method was designed to accommodate a wide variety of sample types, including carbonates, bones, and teeth; silicate rocks and sediments; fresh and marine waters; and biological samples such as blood and urine. The system is highly adaptable and capable of processing up to 60 samples per run at a rate of 32 samples per day on a single chromatographic column during unattended operation.

Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction

Multiple episodes of extensive oceanic anoxia delayed the marine ecosystem recovery from the latest Permian mass extinction. Explaining the ~5-million-year delay in marine biotic recovery following the latest Permian mass extinction, the largest biotic crisis of the Phanerozoic, is a fundamental challenge for both geological and biological sciences. Ocean redox perturbations may have played a critical role in this delayed recovery. However, the lack of quantitative constraints on the details of Early Triassic oceanic anoxia (for example, time, duration, and extent) leaves the links between oceanic conditions and the delayed biotic recovery ambiguous. We report high-resolution U-isotope (δ238U) data from carbonates of the uppermost Permian to lowermost Middle Triassic Zal section (Iran) to characterize the timing and global extent of ocean redox variation during the Early Triassic. Our δ238U record reveals multiple negative shifts during the Early Triassic. Isotope mass-balance modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, peaking during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. Comparisons of the U-, C-, and Sr-isotope records with a modeled seawater PO43− concentration curve for the Early Triassic suggest that elevated marine productivity and enhanced oceanic stratification were likely the immediate causes of expanded oceanic anoxia. The patterns of redox variation documented by the U-isotope record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic. Our results indicate that multiple oscillations in oceanic anoxia modulated the recovery of marine ecosystems following the latest Permian mass extinction.

Congruent Permian-Triassic δ238U records at Panthalassic and Tethyan sites: Confirmation of global-oceanic anoxia and validation of the U-isotope paleoredox proxy

Oceanic anoxia has been proposed as a proximate cause of the endPermian mass extinction (EPME), but evaluation of this hypothesis is hampered by limited detailed knowledge of its timing and extent. The recent development of uranium isotopes (δ238U) in carbonates as a global-ocean redox proxy provides new insights into this problem. Three earlier δ238U studies of Tethyan sections inferred development of extensive oceanic anoxia at the EPME. However, recent work raises concerns that diagenetic alteration may influence the reliability of δ238U records in bulk carbonate sediments. Here, we evaluate this possibility through δ238U analysis of a Permian-Triassic carbonate atoll section from the Panthalassic Ocean (Kamura, Japan) and comparison with existing δ238U profiles from the Tethys Ocean. The Kamura section exhibits a large negative δ238U shift across the EPME horizon identical both in timing and magnitude to those in Tethyan sections, demonstrating beyond a reasonable doubt that the negative seawater δ238U shift at the EPME was a global event, and that it was recorded by shallow carbonate facies globally. The robustness of the U-isotope proxy is further shown by the fact that a common global signal at the EPME was preserved despite major differences in the burial histories of Panthalassic and Tethyan sections, the former having been tectonically subducted and heated to greenschist metamorphic grade, whereas the latter accumulated in stable cratonic settings and experienced milder burial effects. Finally, we use leaching experiments to demonstrate that, although small-scale δ238U heterogeneity is common in both modern and ancient carbonates, it probably does not significantly affect bulk-carbonate δ238U trends. INTRODUCTION The existence of oceanic anoxia during the Permian-Triassic Boundary (PTB) crisis, the most severe biotic crisis in Earth history due to its ~90% species-level mortality rate, was inferred by previous studies on the basis of petrographic, geochemical, and biomarker data (Grice et al., 2005; Isozaki, 1997; Meyer et al., 2008; Wignall and Twitchett, 1996). However, reliance on local redox proxies has resulted in divergent views regarding the timing, extent, and intensity of oceanic anoxia that remain unresolved to date. For example, the onset of anoxia was inferred to be several million years before the end-Permian mass extinction (EPME) in some studies (Isozaki, 1997; Wignall and Twitchett, 1996) but no more than ~100 k.y. prior to the EPME in others (Algeo et al., 2012; Shen et al., 2012), and different studies have argued both for (Grice et al., 2005) and against (Loope et al., 2013) the presence of anoxia in shallow-marine facies. Recent work favors a more complex scenario, characterized by widespread expansion of anoxia at intermediate depths (~200–1000 m) prior to the EPME (Winguth and Winguth, 2012; Feng and Algeo, 2014) followed by episodic upward chemocline excursions into the ocean-surface layer commencing at the EPME (Algeo et al., 2008). The recent development of uranium isotopes (δ238U) in marine carbonates as a globally integrative paleoredox proxy has provided new insights into this problem. Three recent studies of PTB sections document a large (0.4–0.5‰) negative δ238U excursion, suggesting a close relationship between ocean-redox changes and the EPME (Brennecka et al., 2011; Elrick et al., 2017; Lau et al., 2016). However, the reliability and global significance of these records have been challenged on the basis of two issues: (1) potential influences of diagenetic alteration on bulk carbonate δ238U records (Hood et al., 2016; Romaniello et al., 2013), and (2) geographic limitation of existing U-isotope studies of the PTB to the Tethys Ocean, which represented only ~10%–15% of contemporaneous globalocean area. Thus, despite the congruency of previously published Tethyan δ238U records, an independent test of uranium isotopes from a Panthalassic section with a dissimilar diagenetic history is needed to verify oceanic redox changes during the Permian-Triassic transition. Here, we report the first Permian-Triassic carbonate δ238U record from an open Panthalassic Ocean site (Kamura, Japan), evaluate its redox implications relative to existing Tethyan δ238U records, and address concerns about preservation of marine δ238U signals by bulk carbonate sediments. URANIUM ISOTOPE SYSTEM The power of uranium isotopes as a global-ocean redox proxy derives from the long residence time (~500 k.y. for the modern; Dunk et al., 2002) and well-mixed character of U in seawater. Seawater δ238U responds to redox changes because reduction of dissolved hexavalent U [U(VI)] to tetravalent U [U(IV)], which is rapidly removed to anoxic sediments, results in a detectable fractionation of U isotopes, sequestering heavy isotopes in the reduced species (Andersen et al., 2014). Thus, the δ238U of U(VI) dissolved in seawater decreases as the areal extent of bottomwater anoxia increases (Brennecka et al., 2011), providing a direct proxy of global-ocean redox changes. Marine carbonate sediments have been *E-mail: fzhang48@asu.edu; zhff414@hotmail.com GEOLOGY, April 2018; v. 46; no. 4; p. 1–4 | GSA Data Repository item 2018092 | https://doi.org/10.1130/G39695.1 | Published online XX Month 2018

Extensive marine anoxia during the terminal ediacaran period

Extensive marine anoxia in the terminal Ediacaran ocean was associated with the decline of the Ediacara biota. The terminal Ediacaran Period witnessed the decline of the Ediacara biota (which may have included many stem-group animals). To test whether oceanic anoxia might have played a role in this evolutionary event, we measured U isotope compositions (δ238U) in sedimentary carbonates from the Dengying Formation of South China to obtain new constraints on the extent of global redox change during the terminal Ediacaran. We found the most negative carbonate δ238U values yet reported (−0.95 per mil), which were reproduced in two widely spaced coeval sections spanning the terminal Ediacaran Period (551 to 541 million years ago). Mass balance modeling indicates an episode of extensive oceanic anoxia, during which anoxia covered >21% of the seafloor and most U entering the oceans was removed into sediments below anoxic waters. The results suggest that an expansion of oceanic anoxia and temporal-spatial redox heterogeneity, independent of other environmental and ecological factors, may have contributed to the decline of the Ediacara biota and may have also stimulated animal motility.

Addressing the Anthropocene

Environmental context We are entering an epoch – the Anthropocene – in which human activity is changing the face of the planet. To stabilise climate, we may consider deliberate intervention into Earth’s systems, on a global scale. Responsible stewardship requires that we develop a safe, economic and environmentally acceptable means of sequestering CO2 from the atmosphere. Abstract The Anthropocene is an evolutionary transition to an epoch in which human activity has become one of the most important Earth systems. To successfully navigate this transition, we must develop a fully integrated environmental science that anticipates the responses of the human system alongside other Earth systems. Applying this perspective to climate change, the signature global environmental challenge in the early part of the Anthropocene, we analyse the ongoing failures of climate policy and the prospects for serious investment in technologies to remove CO2 from the atmosphere.